A crosslinkable resin coating composition such as a melamine-curing resin coating composition, an isocyanate-curing resin coating composition, an oxidation-curing resin coating composition and the like is coated in the art directly onto a substrate, for example, a metal plate such as a steel plate, aluminum plate, iron plate and the like, wood, inorganic materials other than the above metals, for example, concrete, ceramic, glass and the like, plastics such as polyvinyl chloride, polyethylene terephthalate, polyethylene, nylon and the like, for the purpose of imparting respective functions such as corrosion resistance, appearance, durability, weather resistance, mar resistance and the like to the substrate depending on properties of respective substrates. The above direct coating onto the substrate in the art raises such problems that a selection of an optimum coating method is necessary and troublesome, that generally keeping a uniform coating film thickness is impossible, that coating may easily cause coating drawbacks, that a coating composition may be consumed beyond an amount necessary for coating, that a safe control of a working environment, safety and health is difficult, that difficulty of recovering an old coating film is undesirable from the standpoint of an environmental pollution, and so forth.
Generally, use of a coating composition depending on coating purposes such as a coating method, coating film performances, appearance and the like in the art results various kinds of coating compositions and production of wasteful coating compositions in a large amount, so that unification of various kinds of coating compositions and effective utilization of the coating composition have been demanded in the art.
The present inventors made intensive studies on solving the above problems in the art to find out that the use of a multi-layer application film comprising a top layer formed from a crosslinkable resin coating composition known in the art and an under layer formed from a thermoplastic resin film layer, showing practically no stickiness per se, having a specified elongation and being applied by casting can completely solve the above problems in the art, resulting in accomplishing the present invention.
That is, the present invention relates to:    1. A multi-layer application film formed by laminating at least two resin films, a top layer film (I) being formed from a crosslinkable resin coating composition (A), an under layer film (II) being formed from a thermoplastic resin (B), showing practically no stickiness per se, and having a tensile elongation at breakage in the range of 50 to 1000% as a value measured by the use of a sample of 30 mm in length, 10 mm in width and 0.05 mm in thickness under the conditions of a temperature of −10° C. and a stress rate of 200 mm/min., said multi-layer application film being applied onto a plastic substrate (hereinafter may be referred to as a first invention).    2. A multi-layer application film as in above paragraph 1, wherein the under layer film (II) is formed from a water based urethane resin emulsion.    3. A multi-layer application film as in above paragraph 1 or 2, wherein the water based urethane resin emulsion is prepared by reacting a polyisocyanate compound with an active hydrogen-containing compound reactable with an isocyanate group in the polyisocyanate compound to obtain a hydrophilic group-containing isocyanate-terminating prepolymer, dispersing the prepolymer into water, and by chain-lengthening by use of amines.    4. A method of laminating a multi-layer application film, which method comprises subjecting a multi-layer application film as in any one of above paragraphs 1 to 3 to a thermocompression bonding onto the plastic substrate so that the under layer film (II) may face on the surface of the plastic substrate (hereinafter may be referred to as a second invention).    5. A method of laminating a multi-layer application film, which method comprises simultaneously molding a multi-layer application film as claimed in any one of claims 1 to 3 and a starting material of a plastic substrate so that a under layer film (II) may be formed on the surface of the plastic substrate (hereinafter may be referred to as a third invention).